Adjectives are one of the English parts of speech, although historically they were classed together with the nouns. Certain words that were traditionally considered to be adjectives, including the, this, my, etc., are today usually classed separately, as determiners.

Adjective comes from Latin(nōmen) adjectīvum "additional (noun)",[2] a calque of Ancient Greek: ἐπίθετον (ὄνομα)epítheton (ónoma) "additional (noun)".[3][4] In the grammatical tradition of Latin and Greek, because adjectives were inflected for gender, number, and case like nouns (a process called declension), they were considered a subtype of noun. The words that are today typically called nouns were then called substantive nouns (nōmen substantīvum).[5] The terms noun substantive and noun adjective were formerly used in English, until the word noun came to refer only to the former type, and the second type came to be known simply as adjectives.

A given occurrence of an adjective can generally be classified into one of three kinds of use:

Attributive adjectives are part of the noun phrase headed by the noun they modify; for example, happy is an attributive adjective in "happy people". In some languages, attributive adjectives precede their nouns; in others, they follow their nouns; and in yet others, it depends on the adjective, or on the exact relationship of the adjective to the noun. In English, attributive adjectives usually precede their nouns in simple phrases, but often follow their nouns when the adjective is modified or qualified by a phrase acting as an adverb. For example: "I saw three happy kids", and "I saw three kids happy enough to jump up and down with glee." See also Postpositive adjective.

Predicative adjectives are linked via a copula or other linking mechanism to the noun or pronoun they modify; for example, happy is a predicate adjective in "they are happy" and in "that made me happy." (See also: Predicative expression, Subject complement.)

Nominal adjectives act almost as nouns. One way this can happen is if a noun is elided and an attributive adjective is left behind. In the sentence, "I read two books to them; he preferred the sad book, but she preferred the happy", happy is a nominal adjective, short for "happy one" or "happy book". Another way this can happen is in phrases like "out with the old, in with the new", where "the old" means, "that which is old" or "all that is old", and similarly with "the new". In such cases, the adjective functions either as a mass noun (as in the preceding example) or as a plural count noun, as in "The meek shall inherit the Earth", where "the meek" means "those who are meek" or "all who are meek".

Adjectives feature as a part of speech (word class) in most languages. In some languages, the words that serve the semantic function of adjectives may be categorized together with some other class, such as nouns or verbs. For example, rather than an adjective meaning "big", a language might have a verb that means "to be big", and could then use an attributive verb construction analogous to "big-being house" to express what English expresses as "big house". Such an analysis is possible for the grammar of Standard Chinese, for example.

Different languages do not always use adjectives in exactly the same situations. For example, where English uses to be hungry (hungry being an adjective), Dutch and French use honger hebben and avoir faim respectively (literally "to have hunger", the words for "hunger" being nouns). Similarly, where Hebrew uses the adjective זקוק zaqūq (roughly "in need of"), English uses the verb "to need".

In languages which have adjectives as a word class, they are usually an open class; that is, it is relatively common for new adjectives to be formed via such processes as derivation. However, Bantu languages are well known for having only a small closed class of adjectives, and new adjectives are not easily derived. Similarly, native Japanese adjectives (i-adjectives) are considered a closed class (as are native verbs), although nouns (an open class) may be used in the genitive to convey some adjectival meanings, and there is also the separate open class of adjectival nouns (na-adjectives).

Many languages, including English, distinguish between adjectives, which qualify nouns and pronouns, and adverbs, which mainly modify verbs, adjectives, and other adverbs. Not all languages have exactly this distinction and many languages, including English, have words that can function as both. For example, in English fast is an adjective in "a fast car" (where it qualifies the noun car), but an adverb in "he drove fast" (where it modifies the verb drove).

In Dutch and German, adjectives and adverbs are usually identical in form and many grammarians do not make the distinction, but patterns of inflection can suggest a difference:

Eine kluge neue Idee.

A clever new idea.

Eine klug ausgereifte Idee.

A cleverly developed idea.

A German word like klug ("clever(ly)") takes endings when used as an attributive adjective, but not when used adverbially. (It also takes no endings when used as a predicative adjective: er ist klug, "he is clever".) Whether these are distinct parts of speech or distinct usages of the same part of speech is a question of analysis. It can be noted that while German linguistic terminology distinguishes adverbiale from adjektivische Formen, German refers to both as Eigenschaftswörter ("property words").

Linguists today distinguish determiners from adjectives, considering them to be two separate parts of speech (or lexical categories), but formerly determiners were considered to be adjectives in some of their uses. In English dictionaries, which typically still do not treat determiners as their own part of speech, determiners are often recognizable by being listed both as adjectives and as pronouns. Determiners are words that are neither nouns nor pronouns, yet reference a thing already in context. Determiners generally do this by indicating definiteness (as in a vs. the), quantity (as in one vs. some vs. many), or another such property.

An adjective acts as the head of an adjective phrase or adjectival phrase (AP). In the simplest case, an adjective phrase consists solely of the adjective; more complex adjective phrases may contain one or more adverbs modifying the adjective ("very strong"), or one or more complements (such as "worth several dollars", "full of toys", or "eager to please"). In English, attributive adjective phrases that include complements typically follow the noun that they qualify ("an evildoer devoid of redeeming qualities").

In many languages, including English, it is possible for nouns to modify other nouns. Unlike adjectives, nouns acting as modifiers (called attributive nouns or noun adjuncts) usually are not predicative; a beautiful park is beautiful, but a car park is not "car". The modifier often indicates origin ("Virginia reel"), purpose ("work clothes"), or semantic patient ("man eater"); however, it may generally indicate almost any semantic relationship. It is also common for adjectives to be derived from nouns, as in boyish, birdlike, behavioral (behavioural), famous, manly, angelic, and so on.

Many languages have special verbal forms called participles that can act as noun modifiers (alone or as the head of a phrase). Sometimes participles develop into pure adjectives. Examples of this in English include relieved (the past participle of the verb relieve, used as an adjective in sentences such as "I am so relieved to see you"), spoken (as in "the spoken word"), and going (the present participle of the verb go, used as an adjective in such phrases as "the going rate").

Other constructs that often modify nouns include prepositional phrases (as in "a rebel without a cause"), relative clauses (as in "the man who wasn't there"), and infinitive phrases (as in "a cake to die for"). Some nouns can also take complements such as content clauses (as in "the idea that I would do that"), but these are not commonly considered modifiers. For more information about possible modifiers and dependents of nouns, see Components of noun phrases.

This order may be more rigid in some languages than others; in some, like Spanish, it may only be a default (unmarked) word order, with other orders being permissible.

Due partially to borrowings from French, English has some adjectives that follow the noun as postmodifiers, called postpositive adjectives, as in time immemorial and attorney general. Adjectives may even change meaning depending on whether they precede or follow, as in proper: They live in a proper town (a real town, not a village) vs. They live in the town proper (in the town itself, not in the suburbs). All adjectives can follow nouns in certain constructions, such as tell me something new.

In many languages, some adjectives are comparable. For example, a person may be "polite", but another person may be "more polite", and a third person may be the "most polite" of the three. The word "more" here modifies the adjective "polite" to indicate a comparison is being made, and "most" modifies the adjective to indicate an absolute comparison (a superlative).

Among languages that allow adjectives to be compared, different means are used to indicate comparison. Some languages do not distinguish between comparative and superlative forms.

In English, many adjectives can take the suffixes "-er" and "-est" (sometimes requiring additional letters before the suffix; see forms for far below) to indicate the comparative and superlative forms, respectively:

Another way to convey comparison is by incorporating the words "more" and "most". There is no simple rule to decide which means is correct for any given adjective, however. The general tendency is for simpler adjectives, and those from Anglo-Saxon to take the suffixes, while longer adjectives and those from French, Latin, Greek do not—but sometimes sound of the word is the deciding factor.

Many adjectives do not naturally lend themselves to comparison. For example, some English speakers would argue that it does not make sense to say that one thing is "more ultimate" than another, or that something is "most ultimate", since the word "ultimate" is already absolute in its semantics. Such adjectives are called non-comparable or absolute. Nevertheless, native speakers will frequently play with the raised forms of adjectives of this sort. Although "pregnant" is logically non-comparable (either one is pregnant or not), one may hear a sentence like "She looks more and more pregnant each day". Likewise "extinct" and "equal" appear to be non-comparable, but one might say that a language about which nothing is known is "more extinct" than a well-documented language with surviving literature but no speakers, while George Orwell wrote "All animals are equal, but some are more equal than others". These cases may be viewed as evidence that the base forms of these adjectives are not as absolute in their semantics as is usually thought.

Comparative and superlative forms are also occasionally used for other purposes than comparison. In English comparatives can be used to suggest that a statement is only tentative or tendential: one might say "John is more the shy-and-retiring type," where the comparative "more" is not really comparing him with other people or with other impressions of him, but rather, could be substituting for "on the whole". In Italian, superlatives are frequently used to put strong emphasis on an adjective: Bellissimo means "most beautiful", but is in fact more commonly heard in the sense "extremely beautiful".

Attributive adjectives, and other noun modifiers, may be used either restrictively (helping to identify the noun's referent, hence "restricting" its reference) or non-restrictively (helping to describe an already-identified noun). For example:

"He was a lazy sort, who would avoid a difficult task and fill his working hours with easy ones."

"difficult" is restrictive - it tells us which tasks he avoids, distinguishing these from the easy ones: "Only those tasks that are difficult".

"She had the job of sorting out the mess left by her predecessor, and she performed this difficult task with great acumen."

"difficult" is non-restrictive - we already know which task it was, but the adjective describes it more fully: "The aforementioned task, which (by the way) is difficult"

In some languages, such as Spanish, restrictiveness is consistently marked; for example, in Spanish la tarea difícil means "the difficult task" in the sense of "the task that is difficult" (restrictive), whereas la difícil tarea means "the difficult task" in the sense of "the task, which is difficult" (non-restrictive). In English, restrictiveness is not marked on adjectives, but is marked on relative clauses (the difference between "the man who recognized me was there" and "the man, who recognized me, was there" being one of restrictiveness).

In some languages, adjectives alter their form to reflect the gender, case and number of the noun that they describe. This is called agreement or concord. Usually it takes the form of inflections at the end of the word, as in Latin:

Often a distinction is made here between attributive and predicative usage. Whereas English is an example of a language in which adjectives never agree and French of a language in which they always agree, in German they agree only when used attributively, and in Hungarian only when used predicatively.